1. Field of the Invention
This invention relates to an electrochromic display device (hereinafter called "ECD") using a substance of which the optical absorption characteristic in the visible light region is reversibly varied by the application of electric current, and more particularly to a new counter-electrode for ECD which is disposed opposite a display electrode formed of an electrochromic substance.
2. Description of the Prior Art
FIG. 1 is a sectional view of one device of the prior art. On a transparent glass substrate 1 is formed a transparent electrode 2. On this transparent electrode 2 is provided a layer of an electrochromic substance 3 [such as a tungsten oxide (WO.sub.3) film]. On the remaining part of the transparent electrode 2, an insulating film 4 is formed. A display electrode is composed of these component parts. The other transparent glass substrate 5 which is disposed opposite the glass substrate 1 is formed in the shape of a cap. On the inner wall surface of this glass substrate 5, a conductor 6 is formed by evaporation or spattering. On this conductor 6, a redox layer 7 is formed by evaporation or spattering. These component parts compose a counter-electrode. A white porous plate 8 is interposed between the redox layer 7 and the electrochromic layer 3. The substrates 1, 5 are fastened in position with an adhesive agent 9 in an airtight manner. The empty space enclosed with the substrates 1, 5 is filled with an electrolyte 10. This empty space retains a bubble and, therefore, absorbs possible thermal expansion of the electrolyte 10. Denoted by 12 is a non-woven fabric of glass fibers serving to keep the porous plate 8 in intimate contact with the display electrode.
When the ECD cell thus constructed is operated, since this cell is one type of electrolytic cell, exchange between ion conduction and electron conduction proceeds in both of the display electrode and the counter-electrode. The electrochemical reaction which occurs in the WO.sub.3 film, for example, is known to proceed as shown below. ##EQU1## (wherein, X denotes a number in the range of 0 to 1 and M denotes a metal atom). In the operation of the ECD cell in which the electrochromic layer 3 of the display electrode and the redox layer 7 of the counter-electrode are both made of WO.sub.3 film, the write operation of the display electrode corresponds to the reaction of reduction in the formula (1). In the meantime, the oxidation of the same formula proceeds in the counter-electrode. More specifically, where the WO.sub.3 film used in the counter-electrode as an auxiliary electrode is caused to color (i.e., write operation), the oxidation in the formula (1) proceeds in the redox layer 7.
The data concerning the write operation are shown in FIG. 2 and the data concerning the erase operation are shown in FIG. 3. FIG. 2 and FIG. 3 represent the results of the experiment conducted on an ECD cell constructed as described above and possessed of a display area of 1.5 cm.sup.2 and a counter-electrode (redox layer) surface area of 10 cm.sup.2, by passing a fixed current of 20 mA/cell for 0.5 second (500 m. sec) through, or applying a charge of 10 mC/cell to, the ECD cell and noting the time course change in the potential of the counter-electrode. In the graphs, the continuous line represents the change of the potential and the broken line the change of the current. It is clear from the graphs that when the redox layer of the counter-electrode is formed of colored WO.sub.3 film, the potential of the counter-electrode during the write operation of the cell by use of the fixed current shows the time-course change which is expected in view of the reaction formula (1). After the electrolysis stops, this potential gradually shifts to the equilibrium potential.
The results of another experiment conducted under the same conditions as those mentioned above on a freshly produced ECD cell or an ECD cell which had had the counter-electrode colored by the same method as described above and which thereafter had been preserved at a temperature of 60.degree. C. for 24 hours are shown in FIG. 4 and FIG. 5. It is seen from FIG. 4 that the time-course change of the potential of the counter-electrode during the write operation of the ECD cell contains a point of refraction (A) and eventually reaches a fairly high level. This fact implies that the reaction after the point of refraction is different from the expected reaction (the erase reaction of the colored tungsten) indicated by the reaction formula (1). It is generally known that the oxidation number of the decolorized WO.sub.3 is 6 and that the tungsten atom is not allowed to assume any higher oxidation number than 6. The reaction after the point of refraction, therefore, is inferred to be a reaction to decompose the electrolyte. The decomposition of the electrolyte is thought to induce breakage of the cell. For the ECD cell to provide the repeated display without fail, it is necessary that the cell should retain the color of the WO.sub.3 film in the counter-electrode and enable the counter-electrode to retain its ability to generate the desirable reaction of oxidation.
Incidentally, it is well known that the extent of the coloration of the WO.sub.3 film (the amount of electric charge to be retained) is changed along the course of time. As described above, the extent of coloration of WO.sub.3 is decreased at a rate of about 10% per hour at an elevated temperature of 60.degree. C. and at a rate of about 10% per day at room temperature, and the speed of color erasure depends on the environmental conditions. The ECD cell which uses WO.sub.3 in the redox (active substance) layer of the counter-electrode, therefore, inevitably requires a device of some sort or other capable of enabling the redox layer to retain its capacity for electric charge at a fixed level. Unfortunately, the speed of erasure of the WO.sub.3 film when the film is left standing depends on the temperature. In the case of the display device, which for conventional applications is required to anticipate a wide variety of environments under which it is put to use, it is extremely difficult to retain the capacity for the electric charge at a fixed level under varying conditions.
Various systems claimed to satisfy simultaneously the properties expected of the counter-electrode, namely the ability to pass the electric current in both directions of oxidation and reduction (i.e., an ability to curb polarization) and the ability to retain the equilibrium potential at a fixed level, have been proposed. One proposal is to use an electrode which is molded of a mixture consisting of graphite, a binder, and an electrochromic substance (U.S. Pat. No. 3,827,784, Japanese Laid-open Patent Publication No. 13891/1972, and U.S. Pat. No. 3,978,007). In this proposal, many substances are disclosed as concrete examples of an electrochromic substance (substance capable of exhibiting the phenomenon of electrochromism). They are oxides of transition metals, halogenides, selenides, metal oxo-acid salts, etc. Other relevant proposals are those covering a shaped article molded of an iron complex and a carbon powder (Shigeo Kondo et al: Oct. 31, 1979 issue of Technical Report of Television Study Society, 7), a sintered article of a metal-carbon mixture (Hironosuke Ikeda and Kinya Tada: Electronics Materials, 1980 No. 2, 47), and a press-molded electrode using as an active substance a powdered oxide of transition metal such as manganese oxide and as a combination current collector and carrier an expanded graphite as disclosed by the applicant is his patent applications (Japanese Patent Application No. 168927/1979) (which does not use any binder). All the counter-electrodes covered by the proposals mentioned above are reported to exhibit outstanding properties with respect to the restraint of polarization and the stability of equilibrium potential. These counter-electrodes, however, have a disadvantage, in that because they are press molded, the processes for their manufacture are complicated and their prices are high and the works involved in establishing electric continuity between the electrode interiors and the external components gain in complexity.
Further, the method as disclosed e.g. by Japanese Laid-open Patent Publication No. 13891/1972, which produces a counter-electrode by mixing an active substance and graphite with a binder such as, for example, a thermosetting resin like an epoxy resin or an emulsified or dissolved polymer thereby preparing a paste, spreading this paste on a glass substrate, and subjecting the resultant composite to a thermal treatment, possesses a problem in that the active substance experiences enhanced polarization because it is covered with the binder.
On the other hand, a number of substances possessing a self-film-forming property are utilized in an electrode for a primary battery. For example, it has been proposed to use the substance MnO.sub.2 resulting from the thermal decomposition of a maganese salt in electrodes in the field of primary batteries; to cover the surface of binder particles of the electrode with the thermal decomposition product MnO.sub.2 (Japanese Laid-open Patent Publication No. 58821/1977); to cover the surface of carbon particles with the thermal decomposition product MnO.sub.2 (Japanese Laid-open Patent Publication No. 71628/1977); and to cover the surface of the individual particles of MnO.sub.2 powder with the thermal decomposition product MnO.sub.2 (.beta. type) (Japanese Patent Publication No. 216/1977). In these proposals, it is reported to have exhibited an outstanding property as the electrode for primary battery.